Frequency modulation



Feb. 11, 1941. w PERCIVAL 2,231,854

FREQUENCY MODULATION Filed. Feb. 24, 1939 INVENTOR WILL/AM SPENCERPERCIVAL svvggm ATTO R N EY Patented Feb. 11, 1941 f 2,231, 54

FREQUENCY MODULATION William Spencer Percival, Ealing, London, England,assignor to Electric .& Musical Industries Limited, Hayes, jMid-dlesex,England, a company of .Great Britain Application February 24, 1989,Seria'lNo. 258,141 'In Great Britain February '24, .1938

7 Claims. .(ol. ire-171.5)

This invention relates to thermionic valve cirvide improved means forfrequency modulation cuits and has particular, but not exclusive, of acarrier frequency which is especially suitreference to circuits fortransmitting wide band able where the modulation frequencies comprisefrequency modulated signals, the invention being television or otherwide hand signals. applicable also for other purposes for which itAccording'to a feature of theinvention a thers is desired to obtainoscillations of varying fremionic valve circuit is provided forgenerating quenciesas, for example, for automatic frequency varyingfrequency oscillations comprising a thercorrection in radioreceivers'andtransmitters. mionic valve oscillator including a frequencyIn the field of radio communication by means determining circuit, aplurality of Valves in oas- 9; carrier waves there are threefundamentalcade the :output circuit of the cascade valves 1-0 methods of modulationofa carrier wave, namely, being effectively connected to the inputcircuit amplitude, phase and frequency modulation. At f the cascadevalves for the generation of oscilthe present time the most commonlyemployed lations; and means for applying a controlling method ofmodulation is amplitude modulation, potential for determining thefrequency of the wherein the amplitude of the carrier wave is generatedoscillations. 1.5 varied in sympathy'with the modulation frequen-According to a further feature of the invencies. Now it is known that byusing frequency tion a thermionic valve circuit is provided formodulation instead of amplitude modulation a generating varyingfrequency oscillations commore efiicient transmission can be efiected,thus prising a thermionic valve oscillatorincluding at 25 resulting ineconomy, especially in the operation least twcrvalves each associatedwith va'frequency of .large transmitting stations. In addition, it isdetermining circuit tuned to a different frealso known that frequencymodulation, whereby quency, the outputs of said Valves being effecthe.frequency of the carrier wave is altered in tively connected to theinput of a thermionic response to modulation frequencies, provides anvalve phase-inverting'circuit having separate outimprovement overamplitude modulation in the puts "which are arranged 'to feed backenergy signal-.to-noise ratio of reception, and this is of independentlyto'the input circuits of said valves great importance in thetransmission of 'teleinsuch a manner as to tend -to cause said valvesvision and other signals which occupy wide freto generate oscillationsatthe different frequenquency bands. cies,:said valves being soconnect-ed that they are Known methods of frequency modulation haveforced toproduce oscillations of .a resultant fre- 30 been founddifficult to apply to the wide pass quency, and means for applyingcontrolling band required for television, and there are two, potentialsto said valves so as *to determinethe main reasons for this. In thefirst place, it is resultant frequency of the generated oscillations.not easy to vary the carrier frequency over a In 'order'that the saidinvention maybe clearly sufi-lcien-tly d b nd, d condl it i not aunderstood and readily "carried 'into efiect the 35 simple matter tomaintain a wide pass band for same'wi-ll now be'more fullydescribed withreferthe side bands, it can be shown that theoretically ence to theaccompanying drawing in which the frequency modulation produces aninfinite numsingle figure illustrates a thermionic valve cirber of sidebands. For example, let it berequired cuit arranged in accordance withthe :invention.

40 to vary the frequency of a carrier between 42 In the sin e fi u Ofthedrawing e Screen 40 megacyc-les per second and 48 megacycles pergrid-valve III is provided in'its grid circuit with second at a lowperiodicity to represent a low an inductance 1'01 which, in conjunctionWith frequency component of the modulation signal. the -stray capacitiesof the circuit, forms :a fre- -At the same time let there besuperimposed a d my determining circuit. Similarly, the-valve highmodulation frequency component of say, 3 20 :has an inductance 103connected to its grid 45 megacycles per second, but of a very smallampli- 22,the resonant frequencies of the two frequency tude. Theminimum side bands required for the determining'circuitsfbeing arrangedat or beyond higher frequency-component will be 42 3 megathe oppositeends 0f the band of frequencies over cycles/second :and 484-3megacycles/second dewhich it'isdesiredto vary the frequency of oscil- 50ending on the instantaneous amplitude of the lation. The anodes M and 24of the two valves 5 low frequency component. From "this it will be I 0and 20 are arranged in parallel with a common seen that for television avery wide pass band anode impedance l5 and a suitable de-coupling isrequired in the radio-frequency circuits even resistance 14in-conjunction with a de-coupling if the higher sidcbands are:neglected. condenser 16. ,The anode circuit is coupled by :Itjis theobjector the present invention-to promeansof ;a condenser 48 to the twogrids 52 and which are connected across a part of the inductance I02.Valves 50 and 60 are provided with suitable cathode biasing circuits,namely, resistances 51 and 61 and condensers 58 and 68 respectively.valves 50 and 60 are de-coupled by means of the resistances 59 and 69 inconjunction with a condenser 10'. provided with load resistances, theloadyresistance 55 in the anode of valve 50 being de-coupled from thesource of positive potential by means of resistance II and condenser 12.The .same de-coupling circuit is also used to de-couple the anodeimpedance 65 of valve 60. The anode'54 of valve 50 is connected by meansof condenser 56 to the grid circuit of valve 20 whilst the anode 64 ofvalve 60 is similarly coupled by means of condenser 66 to the grid I2 ofvalve I0. The screening grids of valves l0 and 20 are supplied withsuitable positive potentials by means of resistances 3B and 39, whilecondensers 40 are for the purpose of providing a low impedance atcarrier frequencies between the screening grids and earth. Themodulation potentials are applied between the terminals 35 and 45 inphase opposition. The control grids of valves I0 and 20 are biased byusing a common cathode im pedance I8 Whilst condensers 31 and 41 are forthe purpose of by-passing the impedance I8 at carrier frequencies.

The operation of the circuit can be explained asfollows:

Assume that the valve 20 has a large negativ potential applied to itsscrening grid 23 so that the anode current of the valve is reduced substantially to zero and that the screening grid l3 has a large positivepotential applied to'it. Should the grid of valve I0 become slightlypositive, then the potential applied to the grid of valve 60 is changedin phase by by means of valve I0 and is consequently negative. The valve60 then changes the phase of its input signal by 180, or, in otherwords, the anode 64 becomes positive. This positive signal is fed backto the grid of valve I 0 and since the change of phase of the fed backsignals is exactly 360 the circuit I 0| will oscillate at its resonantfrequency, that is, at a frequency which is at one end of the band offrequencies over which it is desired to modulate the oscillator.Similarly, if the screening grid 23 of the valve 20 has a large positivepotential applied to it and the anode current of valve I0 is reduced tozero, then the circuit I03 will oscillate owing to the feedback producedby valves 20 and 50 at a frequency at the other end of the frequencyband. If both valves are passing current and the ratio of the extremefrequencies of the two circuits I0! and I03 is not too great, thenoscillation will occur at an intermediate frequency. Thus, the effect ofmodulation applied to the terminals 35 and 40 is to make the frequencyvary in sympathy between the extreme limits determined by the frequencydetermining circuits IOI and I03.

It is desirable that the valves I0 and 20 be matched. This is not sonecessary when a com- The screening grids 53 and 63 of Each of thevalves 50 and 60 are, 5

mon cathode impedance I8 is employed which is large at modulationfrequencies but is small at the frequencies of the carrier andsidebands. Such an impedance causes the sum of the currents through thevalves to vary only slightly at modulation frequencies and the effect ofthe mis-matching between the valves is reduced to a second order.

The purpose of resistances I9 and 29 is to prevent the circuitoscillating at very high frequencies although any other suitable meansmay be employed for this purpose.

I claim:

1. A thermionic valve circuit for generating varying frequencyoscillations comprising a thermionic valve oscillator at least twovalves each associated with a frequency determining circuit tuned to adifferent frequency, the outputs of said valves being effectivelyconnected to the input of a thermionic valve phase-inverting circuithaving separate outputs which are arranged to feed back energyindependently to the input circuits of said first two valves in such amanner as to tend to cause said valves to generate oscillations at thedifferent frequencies, said valves. being so connected that they areforced to produce oscillations of a resultant frequency, and means forapplying controlling potentials to said valves so as to determine theresultant frequency of the generated oscillations.

2. A thermionic valve circuit according to claim 1, wherein eachfrequency determining circuit is associated with a pair ofcascade-coupled therminonic valves, the output circuit of each pair ofcascade valves being effectively connected to their respective inputcircuits.

3. A thermionic valve circuit according to claim 1, wherein thecontrolling potentials are applied to a plurality of valves, said valveshaving a common cathode impedance which is high at the frequency of thecontrolling potential or potentials but which is low at the frequency ofthe generated oscillations.

4. In a frequency modulation system a pair of electron discharge valveseach having a cathode, a control electrode and an anode, a circuitbroadly tuned to the frequency of the oscillations to be generatedcoupled to the anodes of said valves, a tuned circuit connected betweenthe control electrode and cathode of each of said valves, an additionalvalve coupling said first named tuned circuit to the control grid of oneof said first named valves, a further valve coupling said first namedtuned circuit to the control grid of the other of said first valves,said additional and further valves serving to reverse the phase ofvoltages supplied thereto and apply the same to said control grids, andmeans for modulating the impedances of said first two valves in phaseopposition at signal frequency.

5. A system as recited in claim 4. wherein the cathodes of said firstpair of valves are tied directly together and. connected to a source ofdirect current by means for maintaining the directcurrent supply to saidcathodes substantially constant.

' '6. In a frequency modulation system a pair of electron dischargevalves each having a cathode, a control grid and an anode, a circuitbroadly tuned to the frequency of the oscillations to be generatedcoupled to the anodes of said valves, circuits tuned to frequencies onopposite sides of the mean frequency to which said first circuit istuned connected between the control grids and cathodes of said valves, athird valve coupling said first named tuned circuit to the control gridof one of said first named valves, a fourth valve coupling said firstnamed tuned circuit to the control grid of the other of said firstvalves, said third and fourth valves serving to apply voltage from saidfirst circuit to said control grids, and means for modulating theimpedances of said first two valves in phase opposition at signalfrequency.

7. A system as recited in claim 6, wherein the cathodes of said firstpair of valves are tied directly together and connected to a source ofdirect current by means for maintaining the directcurrent supply to saidcathodes Substantially 5 constant.

WILLIAM SPENCER PERCIVAL.

CERTIFICATE OF CORRECTION. Patent No. 2,251,85h. February 11, 19141.

WILLIAM SPENCER PERCIVAL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,second column, line 16, claim 1, after "oscillator" insert including-;and that the said Letters re ent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Office.

Signed and sealed this 5th day of May, A. D. 1914.2.

1 Henry Van Arsdale (Seal) Acting Commissioner of Patents.

